This invention relates to a water-absorbent resin and a process for producing the same. This invention relates, more particularly, to a water-insoluble, water-absorbent resin of polyacrylic acid type whose water-absorbing capability is improved.
Water-absorbent resins capable of highly absorbing water as water-absorbent and water-retentive materials have been currently used in such fields as sanitary supplies, medical supplies, cosmetics, agriculture, foods, civil engineering and construction, and household goods, As disclosed in U.S. Pat. Nos. 4,076,663 and 4,090,013, such water-absorbent resins include crosslinked polyethylene oxide, crosslinked polyvinyl alcohol, crosslinked polyacrylic acid and its salt, cellulose-acrylic acid-grafted copolymer and its salts, and hydrolyzates of starch-acrylonitrile-grafted copolymers, of which the water-absorbent resins of polyacrylic acid series have been widely used because of their excellent water-absorbing capability in terms of water absorption and the rate of water absorption.
Such water-absorbent resins of polyacrylic acid type have been utilized in the form of a powder which is produced by mechanically powdering a block of polymer obtained by polymerization in order to increase the rate of water absorption. In consideration of the fact that, in particular, water-absorbent resins with a high rate of water absorption have currently been required, the resins with a larger surface obtained by pulverization are used because the rate of water absorption of water-absorbent resins is proportional to their surface area. However, the pulverization by mechanically powdering means causes water-absorbent resins to be cut in crosslink or to develop a solubilizate fraction, leading to an extremely deteriorated water-absorbing capability such as water absorption capacity and the rate of water absorption. The pulverization also causes the a problem that a film is liable to develop on the particle surface in contact with water, which impedes water penetration and causes powder coagulation (a phenomenon of undissolved lump of flour) when water absorbing, resulting in an insufficient water-absorbing capability. For example, if a commercially available powdery water-absorbent resin is further mechanically powdered, its water-absorbing capability is deteriorated such that water absorption capacity is lowered by about a half and it takes a time as long as 30 minutes to 24 hours to obtain a given water absorption with respect to the rate of water absorption, causing the phenomenon similar to an undissolved lump of flour to be developed. Such powdering to improve the rate of water absorption has been accompanied by the disadvantage that some solubilization occurs or the phenomenon similar to an undissolved lump of flour occurs due to cross-link cutting, both of which extremely deteriorate the water-absorbing capability.
In Japanese Pat. Application laid open No. 168921/1982, a water-absorbent resin produced by coating a highly-water-absorbent macromolecular material such as cross linked polyacrylic acid salt, etc. with a water-soluble resin such as polyethylene glycol, etc. is disclosed which avoids the above-noted disadvantage and which generates no phenomenon similar to an undissolved lump of flour and has a high rate of water absorption. However, when the resin is produced, it involves a uneconomical production process wherein the highly-water-absorbent macromolecular material powder is coated with the water-soluble resin dissolved in a large amount of an organic solvent such as methanol, etc. and then the large amount of the solvent is removed in vacuo, and it is difficult to safely produce the material industrially because of the utilization of a flammable organic solvent. When water was used instead of an organic solvent such as methanol, etc. and the highly-water-absorbent macromolecular material powder was coated with a mixture of water and polyethylene glycol, the resultant water-absorbent resin generated the phenomenon similar to an undissolved lump of flour and failed to increase the rate of water absorption.
In Japanese Pat. Application laid open No. 42602/1983, a method is disclosed which increases the rate of water absorption and prevents the phenomenon similar to an undissolved lump of flour by dispersing a hydrophilic-crosslinked polymer into a dispersing medium such as methanol or ethanol to further crosslink the resin surface with a crosslinking agent such as multivalent metal salt. However, even with that method, an uneconomical production process is required where the resin is surface treated using a large amount of a solvent such as methanol, ethanol, etc., filtered, and then vacuum dried for a long time to remove the solvent, leading to an insufficient improvement of the rate of water absorption and insufficient prevention of the phenomenon similar to an undissolved lump of flour when using only the crosslinking agent such as multivalent metal salt.
Further, a water-absorbent resin is required to have such properties as a high gel strength after water absorbing and an excellent resin fluidity before water absorbing with regard to a workability when the end product using the water-absorbent resin is produced as well as an excellent water absorption and the speed of water absorption. However, no prior water-absorbent resin of polyacrylic acid series has sufficiently satisfied these requirements.